The silicon (Si) used for solar batteries is generally considered required to have a purity of 6N or more, not have various types of metal impurities more than 0.1 mass ppm, and not have boron (B) of at least more than 0.3 mass ppm. Further, as silicon satisfying these conditions, the superhigh purity silicon of a purity of 11N or more used for semiconductor applications is known.
However, this silicon used for semiconductor applications is produced by the so-called Siemen's process of converting metallurgical grade metal silicon of a purity of 98 mass % or so obtained by reducing silica to silicon chloride, next distilling this silicon chloride, then breaking it down by heat. This requires an extremely complicated production process and extremely strict process control and inevitably increases the production cost. For this reason, silicon for semiconductor applications is excessive in quality, too high in production cost, and therefore not suited for solar battery applications where lower cost is being sought along with the growth in demand.
Therefore, in the past as well, attempts have been made to use inexpensive metallurgical grade metal silicon as the material, refine this by a metallurgical method, in particular, the slag refining method of bringing molten silicon into contact with lower density molten slag to make the impurities in the molten silicon move to the molten slag, and thereby inexpensively produce silicon with a purity of 6N or more suitable for solar battery applications.
For example, Japanese Patent Publication (A) No. 56-32319 describes the method of combining a chemical refining stage and vacuum evaporation under predetermined conditions to refine a metallurgical quality silicon material. In particular, Example 3 describes using an induction heating graphite crucible to indirectly heat and melt a silicon material containing 30 mass ppm of boron together with an industrial purity extraction use melt (CaF2+CaO+SiO2) through the graphite crucible, holding this at 1450 to 1500° C. for 30 minutes, extracting the melt, then using an induction heating vacuum melting apparatus to vacuum evaporate this under predetermined conditions to produce silicon with a boron content of 8 mass ppm for solar battery applications.
Further, Japanese Patent Publication (A) No. 58-130114 discloses a method of using an ordinary melt furnace such as an electrical resistance furnace or induction furnace, mixing pulverized state crude silicon before melting with slag comprised of alkali earth metal oxides and/or alkali metal oxides or slag ingredients, then melting them and bringing the molten silicon and molten slag into contact to refine the silicon.
Furthermore, Japanese Patent Publication (A) No. 2003-12317 proposes a method for refining silicon provided with a flux addition step of using an alumina crucible and adding to silicon with a boron concentration of 100 mass ppm or less a flux containing CaO, CaCO3, Na2O, or other basic ingredients and melting the same, a reaction step of blowing oxygen, steam, air, carbon dioxide, or another oxidizing gas into the silicon after the end of the flux addition step, and a flux removal step of removing the flux from the silicon after the end of the reaction step.
Further, “Resources and Materials”, Vol. 118, p. 497 to 505 (2002) reports an experiment using an electrical resistance furnace provided with a quartz crucible and using an SiO2-saturated NaO0.5—CaO—SiO2 type flux containing NaO0.5— which is more strongly basic than alkali earth metal oxides—for separation from molten silicon and thermodynamically considers the distribution behavior of boron.
Further, Japanese Patent Publication (A) No. 2003-213345 describes a method of refining metal using an electromagnetic induction heating apparatus which heats a graphite crucible so as to indirectly heat and melt the material silicon and slag material in this crucible, holding the silicon containing boron or carbon or other impurity elements in a molten state, adding an impurity element absorbing medium comprised mainly of CaO to this molten silicon, using a rotary drive mechanism to mechanically agitate the mixture, further introducing a treatment gas containing steam or other oxidizing ingredients reacting with the impurity elements to form a gaseous compound, and expelling the treatment gas through the impurity element absorbing medium to the outside of the molten silicon.
Furthermore, Japanese Patent Publication (A) No. 2005-247623 describes the method of using a very simple atmospheric melting furnace to heat metal silicon containing impurity boron to its melting point to 2200° C. to melt it, adding to this molten silicon a solid mainly comprised of silicon dioxide (SiO2) and a solid mainly comprised of one or both of a carbonate or carbonate hydrate of an alkali metal to form a molten slag, and bringing these molten silicon and molten slag into contact and, further, jointly using the one-directional solidification method or vacuum melting method as needed, so as to reduce the boron in the silicon down to 0.3 mass ppm or less and furthermore down to 0.1 mass ppm or less.
However, to use inexpensive metallurgical grade metal silicon (below, referred to as “the material silicon”) as a material to produce high purity silicon suitable for solar battery applications by the slag refining method, in particular high purity silicon with a boron content of at least not more than 0.3 mass ppm, inexpensively on an industrial scale, first, the boron in the molten silicon has to be oxidized or else movement to the molten slag cannot be promoted, so use of a strong boron oxidizing slag material is necessary; second, if considering the distribution ratio expressed by the ratio (B/A) of the mass % concentration A of boron in the molten silicon (% A) and mass % concentration B of boron in the molten slag (% B), the slag material has to be used in a relatively high ratio with respect to the material silicon; and, furthermore, third, a large volume of molten silicon and molten slag, in particular molten silicon, commensurate with the industrial scale has to be uniformly and efficiently heated.
Accordingly, in the methods and systems proposed up to now, for example, even when using Na2O or another strongly boron oxidizing slag material for slag refining, to reduce the boron content to 0.3 mass ppm or less, it was necessary to use the slag material usually in a ratio of 1000 mass parts or more, preferably 10000 mass parts or more, with respect to 100 mass parts of the material silicon. Further, if melting the entire amount of the slag material in advance and then bringing it into contact with the molten silicon, there is the problem that the strong oxidizing power cannot be sufficiently utilized at the start of melting of the slag material. If trying to produce silicon in large quantities on an industrial scale, too large a size of a crucible has to be used. Further, uniformly and efficiently heating the molten silicon is also difficult. This is just not practical for working on an industrial scale.